Field of the Invention
The present invention relates to a process for producing a spinnable silica sol material taking into consideration viscosities as process parameters. It further relates to spinnable silica sol material produced according to this process, to a process for producing a fibre or a nonwoven fabric based on such a material and to a fibre obtained thereby or to a nonwoven fabric obtained thereby.
Description of Related Art
Biologically degradable and/or resorbable fibres and nonwoven fabrics which are obtained from a spinnable silica sol material are known in the prior art. The fibres and nonwoven fabrics can be used for example in medical technology and/or human medicine, particularly in wound treatment. For example, WO 2008/086970 A1 describes a silica sol material and its use for producing biologically degradable silica gel materials. The materials such as fibres, nonwoven fabrics, powders, in monolithic form or as a coating can be used for example in medical technology and/or human medicine, especially for wound treatment.
The production of such fibres and nonwoven fabrics can be divided into four steps:                1. Hydrolysis-condensation of a hydrolysable silicon compound        2. Solvent removal        3. Ripening        4. Spinning        
Patent specification DE 196 09 551 C1 discloses by way of example a process for producing biologically degradable/biologically resorbable fibre structures. This document relates to biologically degradable and/or biologically resorbable (continuous) fibres and processes for their production. The (continuous) fibres are obtained by partial or complete hydrolytic condensation of one or more hydrolytically condensable silicon compounds and/or precondensates derived from these. The hydrolytic condensation is carried out by the action of water and optionally in the presence of a catalyst and/or of a solvent and preferably according to a sol-gel process. This partial or complete hydrolytic condensation gives a spinning mass which can be processed in accordance with customary methods to give continuous and/or long and/or short fibres.
According to one embodiment, the process has the following features:                a spinning mass is prepared by hydrolytic (partial) condensation of one or more silicon compounds SiX4 (which is defined above) and/or precondensates derived from these;        the hydrolytic condensation is optionally carried out in the presence of a catalyst and/or of a solvent, by adding water;        an amount of water is used such that the molar ratio SiX4:H2O is between 1:1 and 1:10, preferably between 1:1.5 and 1:2.5;        a phase transfer catalyst or an amount of a water-soluble solvent (LM) or solvent mixture is used such that the molar ratio LM:SiX4 is ≧1, preferably >1;        after hydrolysis has concluded and a dynamic equilibrium has been established, the LM is removed until the resulting mixture has a viscosity between 0.05 and 50 Pa·s, preferably between 0.5 and 2 Pa·s, at room temperature and a shear gradient of 20 s−1;        after removing the solvent, the resulting mixture is subjected to filtration;        after the filtration, the resulting mixture is left to stand until it achieves spinnability;        threads are drawn from the spinning mass and these are optionally dried.        
WO 2008/086970 A1 discloses a similar process, although in this the hydrolysis-condensation step is carried out over a period of at least 16 hours at a temperature of 0° C. to 80° C. with acid catalysis. By means of subsequent evaporation, a single-phase solution with a viscosity in the range from 0.5 to 2 Pa·s at a shear rate of 10 s−1 at 4° C. is said to be produced. This solution is then cooled and it is subjected to a kinetically controlled ripening, during which a homogeneous sol is reportedly formed.
WO 2008/148384 A1 discloses a further process for producing a polyethoxysilane (PES) material. The material is obtained by
(a) carrying out a first hydrolysis-condensation reaction (HCR) of at most one radical X from one or more different Si compounds of the formula SiX4 in which the radicals X are identical or different, and are hydroxy, hydrogen or ethoxy (EtO), with acidic catalysis at an initial pH of 0 to <7, in the presence of ethanol (EtOH) or an ethanol/water mixture as solvent, over a period of 1 to 24 h at a temperature of 0° C. to 78° C. (boiling point of ethanol),
(b) carrying out a second HCR of the material obtained in step (a) with the simultaneous removal of the solvent by successive evaporation in a gas-diffusion-tight container at a pressure of 100 to 1013 mbar, preferably at a slight underpressure of 300 mbar to 800 mbar, a temperature of 50-78° C., preferably of about 70° C., to the point of a drastic viscosity increase (at a shear rate of 10 s−1 at 4° C.) to 0.5 to 2, preferably 1 Pa·s, to constant weight and to the point of the formation of a cyclotetrasiloxane of the general formula ((SiO(OH)0.75(OET)1.25× 1/64 H2O)4 and of molar mass of 4*about 114 g=about 456 g;
(c) cooling this PES material in a closed, preferably gas-diffusion-tight container in a period from a few (2 to 5) minutes to a few (0.2 to 5, preferably 0.5) hours, and
(d) converting the PES material obtained from (c) by means of a third HCR into an rPES material.
WO 2009/077104 A1 discloses a similar process to that in WO 2008/148384 A1 in which the evaporation takes place in a closed apparatus optionally by continuously introducing a chemically inert entrainer gas stream.
In all of the cited disclosures, it is indicated that the ripened silica sol material should have a viscosity in a preferred range in order to be able to spin it.
However, all of the stated processes from the prior art can supply a specific statement regarding the viscosity of the spinnable material with on-spec loss factor after ripening, but give a relatively wide range for the viscosity (between 30 and 100 Pa·s). This has the following disadvantages for the synthesis and spinning of the sol:                In contrast to the measurement of the dynamic viscosity, the measurement of the loss factor is realized not without considerable expenditure inline or online Consequently, it is essential to measure the loss factor offline. Since the viscosity of the sol changes within very small time intervals depending on the selected ripening temperature, especially at the end of ripening, this often leads to loss of the mixture. This loss of the sol can only be prevented through increased measurement effort and personnel expenditure resulting therefrom.        The operating parameters of the spinning plant must be adapted for each new sol in order to obtain an on-spec nonwoven fabric which is biodegradable.        Adaptation of the operating parameters leads to a time delay (reduction in space-time yield) and also to loss of spinnable sol.        If the spinnable material has a high viscosity (>65 Pa·s), then the space-time yield drops during spinning if on-spec nonwoven fabrics are still desired.        